الفهرس 
Title : Retention strength of polyetheretherketone crowns Vs. zirconia crowns restoring molar teeth An in vitro study
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Abstract
Aim: This study was designed to evaluate the retention strength of monolithic CAD/CAM PEEK crowns versus that of CAD/CAM monolithic 5Y-TZP zirconia crowns, both resin-cemented to epoxy resin die abutments resembling lower molar tooth, after thermomechanical aging simulating 3 months of clinical service. Methodology: 22 identical epoxy dies, resembling the preparation of a lower second molar abutment for full-coverage crown (4 mm height, 6{u00B0} axial convergence and 1 mm-thick deep-chamfer finish line), were produced by replicating one master stainless steel die via 22 vinylpolysiloxane impressions. The 22 epoxy dies were randomly equally distributed into 2 groups, 11 dies each; control group 2Zr3 to receive CAD/CAM milled monolithic 5Y-TZP crowns (KATANA{u2122} Zirconia ML), and intervention group 2P3 to receive CAD/CAM milled monolithic PEEK crowns (breCAM.BioHPP{u2122}). Two opposing, laterally projecting arms (mesial and distal) were included in the design of all crowns for the retention test. The fitting surfaces of all crowns of both groups were sandblasted with 110-æm alumina particles.All of the crowns of both groups were cemented over their corresponding epoxy dies using the hand-mixed dual-cured, self-adhesive resin cement (RelyX{u2122} U200 Clicker{u2122} Dispenser) (3-kg seating force). Samples were stored in a water incubator (37 {u00B0}C) for 24 hours; then, subjected to mechanical aging (37,500 cycles, 49 N load) with simultaneous thermocycling (300 cycles, 5-55 {u00B0}C, 12 s dwell time).Retention strength was evaluated through the pull-off test performed via a universal testing machine (UTM), by vertically pulling-up the crowns from their dies along their vertical axes.The mean dislodgement force (retention strength) for each group was calculated, and statistically compared using Student t-test. Statistical analysis was performed with three-way ANOVA. Pair-wise comparison of groups was made with the Tukey test. Pearson{u2019}s Chi-Square test was used to analyze the failure modes. Results: PEEK crowns group (P) has demonstrated statistically insignificant higher retention strength (171.28 N ± 26.30) than zirconia (Zr) crowns group (150.40 N ± 24.40).2Mixed3 failure mode was the predominant in both groups, followed by 2cement on-crown3 failure mode.Though, the material had a significant effect on the percentage of occurrence of failure modes